Alkylation of aromatic compounds with a C2 to C4 olefin and transalkylation of polyalkylaromatic compounds are two common reactions for producing monoalkylated aromatic compounds. Examples of these two reactions that are practiced industrially to produce ethylbenzene are the alkylation of benzene with ethylene and the transalkylation of benzene and a diethylbenzene. A simplified summary of the alkylation reaction and its common product and byproducts is given below:

In addition to these byproducts, the C2-C4 olefin can dimerize to form a C4-C8 olefin or oligomerize to form a C6-C12 olefin. These higher olefins can in turn react with benzene to form alkylbenzenes having alkyl groups with 4 to 12 carbon atoms, such as butylbenzenes, hexylbenzenes, octylbenzenes, and dodecylbenzenes. These heavy alkylbenzenes can themselves be further alkylated to form other heavy polyalkylated benzenes.
Although the formation of the diethylbenzene, triethylbenzene, and tetraethylbenzene (TeEB) isomers might, at first glance, be viewed as byproducts that represent a reduction in the efficient utilization of ethylene, in fact each can be readily transalkylated by benzene to produce ethylbenzene, as shown below:

Combining alkylation and transalkylation can thus maximize ethylbenzene production. Such a combination can be carried out in a process having two reaction zones, one for alkylation and the other for transalkylation, or in a process having a single reaction zone in which alkylation and transalkylation both occur. In many cases, a single reaction zone is preferred over two reaction zones because of the savings in capital investment.
One disadvantage of alkylation-transalkylation processes, regardless of whether the alkylation and transalkylation reactions occur in the same or separate reaction zones, is that byproduct 1,1-diphenylethane (1,1-DPE) can not be readily converted to ethylbenzene by alkylation or transalkylation. Similarly, byproduct alkylbenzenes formed from a dimerized or oligomerized olefin, such as butylbenzenes, hexylbenzenes, octylbenzenes, and dodecylbenzenes can not be converted to ethylbenzene by alkylation or transalkylation. These byproducts represent a reduction in ethylene utilization efficiency and a loss of ethylene. In fact, the byproduction of 1,1-DPE, as well as of the heavier polyethylated benzenes other than diethylbenzene and triethylbenzene, and of the butylbenzenes and octylbenzenes represents virtually all of the reduction in the ethylene utilization efficiency and a loss of benzene as well. As used herein, the term “heavies” refers to polyalkyl aromatics other than dialkyl and trialkyl and tetraalkyl aromatics where the alkyl group has the same number of carbon atoms as the feed olefin, to alkylaromatics formed from dimerized or oligomerized olefins such as butylbenzenes when the olefin is ethylene, and to other even heavier alkylation and transalkylation byproducts including diphenylalkanes (DPA) and alkylated diarylalkanes (DAAs), such as diphenylethanes (DPEs), alkylated diarylethanes (DAEs), diphenylpropane (DPP), and alkylated diarylpropanes (DAPs). The current minimum requirement for combination processes is that 1,1-DPE be not more than 1.0 wt-% relative to ethylbenzene. The formation of 1,1-DPE itself is assuming added importance and significance in view of the expectation in some areas of near-term minimum standards for the content of 1,1-DPE of not more than 0.5 wt-%.
In reaction zones where alkylation and transalkylation occur to produce a monoalkylated aromatic, a key operating variable is the molar ratio of aryl groups per alkyl group. The numerator of this ratio is the number of moles of aryl groups passing through the reaction zone during a specified period of time. The number of moles of aryl groups is the sum of all aryl groups, regardless of the compound in which the aryl group happens to be. In the context of ethylbenzene production, for example, one mole of benzene, one mole of ethylbenzene, and one mole of diethylbenzene each contribute one mole of aryl group to the sum of aryl groups. The denominator of this ratio is the number of moles of alkyl groups that have the same number of carbon atoms as that of the alkyl group on the desired monoalkylated aromatic and which pass through the reaction zone during the same specified period of time. The number of moles of alkyl groups is the sum of all alkyl and alkenyl groups with the same number of carbon atoms as that of the alkyl group on the desired monoalkylated aromatic, regardless of the compound in which the alkyl or alkyl group happens to be, except that paraffins are not included. In the context of ethylbenzene production, the number of moles of ethyl groups is the sum of all ethyl and ethenyl groups, regardless of the compound in which the ethyl or ethenyl group happens to be, except that paraffins, such as ethane, propane, n-butane, isobutane, pentanes, and higher paraffins are excluded from the computation of the number of moles of ethyl groups. For example, one mole of ethylene and one mole of ethylbenzene each contribute one mole of ethyl group to the sum of ethyl groups, whereas one mole of diethylbenzene contributes two moles of ethyl groups and one mole of triethylbenzene contributes three moles of ethyl groups. Butylbenzene and octylbenzene contribute no moles of ethyl groups
In response to the hydrocarbon processing industry's demands for lower molar ratios of aryl groups per alkyl group and more efficient utilization of feed olefins, improved processes for the production of alkylbenzenes are sought.